Polymeric materials, including thermoplastics and thermosets, are used extensively in automotive vehicles and for other purposes. They are light and relatively easy to fashion into complex parts, and are therefore preferred instead of metals in many instances. However a problem with some polymers is salt stress (induced) corrosion cracking (SSCC), where a part under stress undergoes accelerated corrosion when under stress and in contact with inorganic salts. This often results in cracking and premature failure of the part.
There also is a current desire in the automotive field to have high temperature resistant structures since temperatures higher than 150° C. are often reached in under-hood areas of automobiles. When plastic parts are exposed to such high temperatures for a prolonged period, such as in automotive under-the-hood applications or in electrical/electronics applications, the mechanical properties generally tend to decrease due to the thermo-oxidation of the polymer. This phenomenon is called heat aging.
Polyamides such as polyamide 6,6, polyamide 6, polyamide 6,10 and polyamide 6,12 have been made into and used as vehicular parts and other types of parts. While it has been reported that polyamides 6,10 and 6,12 are more resistant to SSCC (see for instance Japanese Patent 3271325B2), all of these polyamides are prone to SSCC in such uses, because for instance, various sections of vehicles and their components are sometimes exposed to salts, for example salts such as sodium chloride or calcium chloride used to melt snow and ice in colder climates. Corrosion of metallic parts such as fittings and frame components made from steel and various iron based alloys in contact with water and road salts can also lead to formation of salts. These salts, in turn, can attack the polyamide parts making them susceptible to SSCC. Thus polyamide compositions with better resistance to SSCC are desired.
U.S. Pat. No. 4,076,664 discloses a terpolyamide resin that has favorable resistance to zinc chloride.
European patent application 0272503 discloses a molding polyamide resin comprising poly(m-xylylenesebacamide) (PA MXD10) and a crystalline polyamide having a melting point about 20-30° C. higher than that of PA MXD10.
US 2005/0234180 discloses a resin molded article having an excellent snow melting salt resistance, said article comprising 1 to 60% by weight of aromatic polyamide resin.
In an attempt to improve heat aging characteristics, it has been the conventional practice to add heat stabilizers (also referred as antioxidants) to thermoplastic polyamide resins. Examples of such heat stabilizers include hindered phenol antioxidants, amine antioxidants and phosphorus-based antioxidants. For polyamide compositions, three types of heat stabilizers are conventionally used to retain the mechanical properties of the composition upon exposure to high temperatures. One is the use of phenolic antioxidants optionally combined with a phosphorus based synergist as previously mentioned, the use of aromatic amines optionally combined with a phosphorus based synergist and the third one is the use of copper salts and derivatives. Phenolic antioxidants are known to improve the mechanical/physical properties of the thermoplastic composition up to an aging temperature of 120° C.
U.S. Pat. No. 5,965,652 discloses a thermally stable polyamide molding composition containing colloidal copper formed in situ. However, the disclosed compositions exhibit retention of impact strength only for a heat aging at 140° C.
GB patent 839,067 discloses a polyamide composition comprising a copper salt and a halide of a strong organic base.
EP 1041109 discloses a polyamide composition comprising a polyamide resin, a polyhydric alcohol having a melting point of 150 to 280° C., that has good fluidity and mechanical strength and is useful in injection welding techniques.
US Patent publication 2010/0029819 discloses thermoplastic articles having high heat stability including a thermoplastic composition including a thermoplastic resin, one or more polyhydric alcohols having more than two hydroxyl groups.
US Patent publication 2010/0271305 discloses thermoplastic articles having high heat stability including a thermoplastic composition including a polyamide resin, one or more polyhydric alcohols having more than two hydroxyl groups; one or more co-stabilizer(s) selected from the group consisting of secondary aryl amines and hindered amine light stabilizers, and mixtures thereof; and 10 to 60 weight percent of one or more reinforcement agents.
Unfortunately, with the existing technologies, molded articles based on polyamide compositions either suffer from an unacceptable deterioration of their mechanical properties upon long-term high temperature exposure or they lack the salt resistance properties necessary for many automotive applications wherein parts become exposed to road salt. Needed are thermoplastic polyamide compositions that have improved long-term heat resistance and salt resistance.